Electrophotographic photoreceptors which exhibit a light sensitivity in visible light range have been provided for the purpose of application to copying machine, photoprinter and the like. As such electrophotographic photoreceptors there have been widely used photoreceptors essentially comprising an inorganic photoconductive substance such as sellenium, zinc oxide and cadmium sulfide. However, these inorganic photoreceptors cannot always satisfy the properties required for electrophotographic photoreceptors for copying machine or the like, such as light sensitivity, thermal stability, humidity resistance and durability.
For example, selenium photoreceptors are subject to crystallization by heat or stain of fingerprint given when touched with hand and thus are susceptible to electrophotographic photoreceptors for this application.
Electrophotographic photoreceptors comprising cadmium sulfide are poor in humidity resistance and durability. Electrophotographic photoreceptors comprising zinc oxide leave to be desired in film strength or other durability. Furthermore, selenium and cadmium sulfide are toxic and thus give a great restriction in preparation and handling.
In recent years, electrophotographic photoreceptors comprising various organic substances have been studied, developed and partily put into practical use to overcome these disadvantages of photoreceptors comprising inorganic substances. Examples of such electrophotographic photoreceptors include electrophotographic photoreceptors comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-one as described in U.S. Pat. No. 3,484,237, poly-N-vinylcarbazole sensitized with a pyririum salt dye as described in JP-B-48-25658 (the term "JP-B" as used herein means an "examined Japanese patent publication"), electrophotographic photoreceptors comprising as a main component an organic pigment as described in JP-A-47-37543 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and electrophotographic photoreceptors comprising as a main component an eutectic complex of a dye and a resin as described in JP-A-47-10785.
However, although these photoreceptors can overcome the above mentioned disadvantages to some extent, they are generally disadvantageous in that they exhibit a low light sensitivity and are not suited for repeated use. Thus, these photoreceptors cannot sufficiently satisfy the above mentioned properties.
In order to overcome these disadvantages, an electrophotographic photoreceptor has been proposed comprising a photoconductive layer having a charge-generating effect and a charge-transporting effect accomplished by separate substances. Such a separate effect type electrophotographic photoreceptor has become a major target of the current study. In the study of such a separate effect type electrophotographic photoreceptor, the range of materials to be selected has been widened. This has enabled the improvement in sensitivity, durability and other properties of the electrophotographic photoreceptors. Furthermore, the separate effect type electrophotographic photoreceptor is advantageous in that substances suitable for coating of film of electrophotographic photoreceptor can be selected from a wide range of substances.
As effective organic charge-generating substances to be incorporated in the charge-generating layer in such a separate effect type electrophotographic photoreceptor there have been developed various organic dyes and organic pigments. Examples of such organic dyes and pigments include azo pigments, perylene pigments, polycyclic quinone pigments and squaric methine dyes having various structures.
However, although these pigments exhibit a relatively excellent sensitivity in a short or middle wavelength range, they exhibit a poor sensitivity in a long wavelength range and thus can hardly be used in laser printers employing a semiconductor laser which is expected to provide a high reliability. At present, the vibration wavelength of a potassium-aluminum-arsenic light-emitting element which is widely used for semiconductor laser is 750 nm or higher.
A phthalocyanine compound, which is one of organic photoconductive materials, is known to have an extended sensitivity range in a long wavelength range as compared to the above mentioned pigments and dyes. However, such a phthalocyanine compound leaves to be desired in electrophotographic properties such as sensitivity and chargeability. In order to overcome these defects, various improvements have been made. For example, various central metals have been used for phthalocyanine. Furthermore, various crystal forms have been developed. Various crystal forms of phthalocyanines have been found in the process during which an unstable .alpha.-type phthalocyanine is converted to a stable .beta.-type phthalocyanine. For example, .epsilon.-type copper-containing phthalocyanine, X-type metal-free phthalocyanine, and m-type titanyl phthalocyanine have been known. Although these phthalocyanines exhibit sensitivity in a long wavelength range, their sensitivity is not sufficient for copying machine or photoprinter. They are also disadvantageous in that they lack potential stability or show a large residual potential after repeated use. Thus, these phthalocyanines cannot be put into practical use.
On the other hand, in order to improve the sensitivity of an electrophotographic photoreceptor comprising a phthalocyanine pigment, it has been proposed to incorporate a charge-transporting compound such as hydrazone compound and oxazole compound or an electron attractive compound such as tetranitrofluorene and trinitrofluorene therein. This approach can provide a sensitizing effect but cannot provide a sufficient sensitizing effect. Furthermore, an electrophotographic photoreceptor comprising such an additive exhibits a drop in chargeability or shows a drop in potential stability and sensitivity and a rise in residual potential after repeated use and thus cannot be put into practical use. Moreover, such an electron attractive compound is toxic and thus cannot be put into practical use.
It has therefore been desired to provide an electrophotographic photoreceptor which is highly sensitive to light of a wavelength of 750 nm or more and exhibits a high potential stability, small residual potential and small drop in sensitivity.